Method of selectively exposing the grains of a mixed grain photographic emulsion



Ap 1952 B. H. CARROLL ET AL 3 METHOD OF SELECTIVELY EXPOSING THE GRAINS OF A MIXED GRAIN PHOTOGRAPHIC EMULSION Filed Oct. 30, 1948 FIG.1.'

FIGZ.

%NSMISSION WAVE LENGTH OPTICAL DENSITY FIG. 3.

W TRANSM/JS/ON WA VE LENGTH OPT I CAL DEN-Tl TY BURT H. CARROLL I VESLEY T. HANSON, JR.

IN V EN TORS TTORNEYS Patented Apr. 8, 1952 METHOD OF SELECTIVELY EXPOSING THE GRAINS OF A MIXED GRAIN PHOTO- GRAPHIC EMULSION Burt H. Carroll and Wesley T. Hanson, J r., Rochester, N. Y., assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Application October 30, 1948, Serial No. 57,592

3 Claims.

This invention relates to photography and particularly to a method of sensitizing and processing mixed grain photographic emulsions.

It is a well-known fact that many optical sensitizing dyes cannot be used in multi-layer photographic coatings for color processes because they diiiuse from the emulsion layer in which they are placed into the adjacent emulsions, thus destroying color separation. When sensitized .emulsions are mixed for what are commonly known as mixed grain processes, the tendency to diffuse is much more serious. It is obvious that this should be the case: In multi-layer films, a thin coating of wet emulsion is applied over another emulsion layer, chilled so that it sets within a few seconds of application and maintained at a low temperature in the set condition during the period of drying, which is usually less than an hour. Diffusion of dye from sensitized to unsensitized or differently sensitized grains must take place through distances of the order of the thickness of the emulsion coatings and at low temperature. In the mixed grain processes, sensitized and unsensitized grains are mixed in the liquid emulsion at temperatures of the order of 35 to 40 C. If the mixed grain emulsion were to be allowed to stand indefinitely, it would in all cases reach an'equilibrium in which the-distribution of all dyes was uniform over the surface of .all grains, except for such minor differences as would be produced by the differences in composition (for example, proportion of silver iodide) of the grains. In practice, only a rate of. diffusion low enough to be negligible during the period of coating and drying can be expected. Very few optical sensitizing dyes meet this requirement.

The effect of diffusion of small amounts of sensitizing dyes is especially serious in reversal processes in which the emulsion layers are selectively re-exposed and color-developed. In this type of process, there may be a normal development of the latent images to metallic silver, followed by a uniform exposure of the film to red light which makes the remaining red-sensitive grains developable. These are then developed to a cyan image. The film is then exposed to green light and the grains which have been made developable are developed to a magenta image.

The remaining grains may be exposed to blue light and developed to a yellow image. Under these conditions, the diffusion of a very small amount of sensitizer is serious because even weak sensitization of the grains to a color difierent from the desired one, for example, red sensitization of green-sensitive grains, will make a considerable proportion of them developable when the film is given the heavy uniform exposure necessary to make all grains of another type developable. Using the same example, many greensensitized grains might be fogged by the uniform flash with red light and hence developed cyan instead of magenta, if re d sensitizer had diffused. The effect is the more pronounced because a small amount of sensitizing dye gives a sensitizing effect more than proportional to its concentration when compared with the effect of the amounts usual for full sensitization.

It is therefore an object .of the present invention to provide a novel method of sensitizing mixed grain emulsions. A further object is to provide a procedure for processing mixed grain emulsions sensitized according to our method. A

still further object is to provide a method for minimizing the effect of sensitizing dye diffusion in mixed grain processes.

These objects are accomplished by using in a mixed grain process an optical sensitizing dye which diffuses only slightly and which produces a different sensitizing maximum on the silver halide grains to which it diffuses than on the originally sensitized grains, and by excluding light of the wave length to which the difiused dye sensitizes, in exposing the originally sensitized grains.

The accompanying drawing shows graphs lllustrating the effect of sensitizing by the diffusing dye.

Our process depends upon the use of optical sensitizing dyes having spectral absorption and sensitizations which vary with the concentration of the dye. It is known that certain optical sensitizing dyes sensitize at low concentration with a maximum of sensitivity displaced about 30 to 50 millimicrons toward longer wave lengths from the maximum of absorption in alcoholic solution. At higher concentrations a new sensitizing maximum develops at still longer wave length, usually about ,50 ,millimicrons from the f rst, although displacements of 30 to millim cro s have bee observed. This new maximum is believed to correspond to a new state of aggregation of the dye although this theory is not essential to our invention.

Such sensitizing dyes are normally used in concentrations which give the aggregate or second maximum at thelonger wave length. If such a dye diffuses slightly, the result is a sensitization of the other grains by a low concentration of. the dye. It accordingly sensitizes in the region of the maximum at shorter wave length when it diffuses. When such dye is used in a mixed grain process of the reversal type, it is possible to make the uniform reversal exposure by means of a filter which absorbs the light of the region to which the grains are sensitized by the diffused dye. This is possible because of the difference of approximately 50 millimicrons between the sensitizing maximum of the dye at high concentration and at low concentration. The exposure therefore may be limited to the grains which it was intended to expose and excluded from those grains which have been unintentionally sensitized by the diffusing dye.

For example, one of the optical sensitizing dyes used in our process has its absorption maximum in alcohol at 575 millimicrons. At low concentration it sensitizes with a maximum at 600 millimicrons. At higher concentrations, for example, at 0.13 gram per mole of silver halide in a moderate speed emulsion (about 28 milligrams per liter of emulsion) it sensitizes with a maximum at 675 millimicrons. If this dye is used in a mixed grain color process, the diffusion from the red-sensitive grains to the others is slight, but nevertheless sufficient to degrade the color rendering if the red light exposure after the first development in the case of a reversal process is made with a filter of the type commonly used for tricolor separations, that is, one transmitting all wave lengths of the visible spectrum greater than 600 millimicrons. In this case, an appreciable proportion of the grains originally unsensi- .difiused dye is negligible at 650 millimicrons or longer, while the grains originally sensitized with the dye .have strong sensitivity at 675 millimicrons.

Dyes of the following classes are suitable for use according to our invention (1) Carbocyanines derived from p-naphtho- -thiazole substituted in the central position of the chain by alkyl or aryl groups (Brooker U. S. Patent 1,846,301). These include 3,3 dimethyl 9 ethyl 4,5,4',5' dibenzthiacarbocyanine chloride 3,3 dimethyl 9 phenyl 4,5,4,5' dibenzthiacarbocyanine bromide 3,3,9 triethyl 4,5,4',5' dibenzthiacarbocyanine bromide.

Corresponding dyes from B-naphthiazole nuclei substituted in the nucleus by halogen, alkyl or Ialkoxy may also be used and the alkyl groups on the 3-positions may be replaced by hydroxyalkyl or'carboxyalkyl and the aryl group in the 9-position may be substituted by alkyl, halogen, or alkoxy. These are red sensitizers.

(2) Carbocyanines derived from substituted benzothiazole or benzoselenazole nuclei substituted in the central position of the chain by alkyl or aryl groups, for example, 3,3',9-triethyl-5,5'- dichloroselenacarbocyanine bromide (made by the method of White U. S. Patent, 1,990,681, using 5 chloro 2 methylbenzoselenazole instead of z-methylbenzoselenazole). These are red sensitizers.

(3) 3,3 dimethyl 8,10 aryloxythiacarbocyanines and oxacarbocyanines (Brooker and White U. S. application Serial No. 519,354, now Patent No. 2,478,366). These include 3,3 dimethyl 8,10 m toloxythiacarbocyanine bromide 3,3 dimethyl 8,10 m toloxyoxacarbocyanine bromide These are red and green sensitizers, respectively.

(4) Carbocyanines derived from S-phenylbenzoxazole or 5-(p-to1yl)-benzoxazole, substituted in the central position of the chain by alkyl or aryl groups, for example, 3,3,9-triethyl-5,5'-dip-tolyloxacarbocyanine bromide (see British Patent 496,116). These are green sensitizers.

(5) Certain 2-cyanines, for example, 1,3-diethyl-5-phenyl-6-methoxy thia-2'-cyanine bromide (Van Zandt & Brooker U. S. application Serial No. 711,816, now Patent No. 2,515,913), and 1',3 diethyl 4,5 benz-6'-methylthia-2'-cyanine bromide (Brooker U. S. Patent 1,935,696). These are green sensitizers.

The optical sensitizing dyes are used according to our invention at a concentration of from 7 to 50 milligrams of sensitizing dye per liter of silver halide emulsion containing about 0.25 gram mole of silver halide per liter of emulsion. We prefer to use from 60 to milligrams of sensitizing dye per gram mole of silver halide.

As we have stated, the optical sensitizing dyes used according to our invention will produce a different sensitizing maximum at high concentration than at low concentration. We believe that this difierence is due to an aggregate form of the sensitizing dye at high concentration and the molecular form of the dye at low concentration. By high concentration we mean the concentration of sensitizing dye which is present on the silver halide grain when used in the range of from 30 milligrams of sensitizing dye per gram mole of silver halide upward. The effective amount of dye depends to some extent on the type of emulsion and composition of the dye itself, as indicated by the following examples. By low concentration we mean the form which the dye takes when it diffuses through a silver halide emulsion and becomes attached to silver halide grains which were not present in the emulsion when the sensitizing dye was added, as by mixing sensitized and unsensitized portions of the emulsion.

The sensitizing maxima produced at the different concentrations for typical dyes used according to our invention are indicated by the following table:

Our invention will now be illustrated by reference to the following specific examples.

Eazample 1 A gelatino-silver bromoiodide emulsion containing 0.24 gram mole of silver bromoiodide per liter was divided into two equal portions. To one portion there was added an alcoholic solution of the red sensitizer 3,3'-dimethyl-9-ethyl- 4,5,4-dibenzthiacarbocyanine chloride, 20 milligrams of dye being added per liter of emulsion. The emulsion was heated to 50 C. and cooled to the coating temperature of approximately 38 C. To the second portion of emulsion there was added 20 milligrams of the green sensitizer 1',3- diethyl 5 phenyl 6 methoxy thia 2' cyanine bromide in alcoholic solution per liter of emulsion. This portion was also heated to 50 C., cooled to 38 C'., then mixed with the other portion of the emulsion and coated.

After drying, the coating was exposed to a color chart and was processed as follows, all of the solutions being at 68 F'.:

(1) Prehardened in the following solution for 3 minutes:

Sodium hexametaphosphate grams 0.5

Sodium bisulfite do Formaldehyde (40% solution) cc 27 Sodium carbonate "grams" 10 Sodium sulfate do 100 Potassium bromide do 2.5

Water to 1 liter.

(2) Washed for four minutes.

(3) Developed in a black and white developer of the following composition for 8 minutes:

Grams Sodium hexametaphosphate 0.5 Sodium sulfite 40 N-methyl-p-aminophenol sulfate 5.5 Hydroquinone 2.2 Sodium metaborate 25 Sodium thiocyanate 1 0.45 Potassium bromide 4.5 6-nitrobenzimidazole .02

Water to 1 liter.

(4) Washed for 10 minutes.

(5) Re-exposed to red light through a Wratten No. 70 filter.

(6) Developed in a cyan developer of the following composition for minutes:

Grams Sodium hexametaphosphate 0.5 Sodium sulfite 5 2 amino 5 diethylaminotoluene hydrochloride 2.8 Sodium carbonate 15 Potassium bromide 1.5 G-nitrobenzimidazole 0.15- 2,4-dichloro-5-p-toluene sulfonamino 1 naphthol 0.4 Sdoium hydroxide 2 Water to 1 liter.

Sodium hydroxide 2 Water to 1 liter.

(12) Washed for 10 minutes. (13) Treated for four minutes in the following ferricyanide bath:

Grams SodiumIhexametaphosphate 1 Potassium ferricyanide .80 Potassium bromide 40 Water to 1 liter.

(.14) Fixed for two minutes in the following bath:

' Grams Sodium hexametaphosphate 10 Sodium sulfite 10 Sodium thiosulfate 320 Water to 1 liter.

(15) Washed for 20 minutes.

Using this procedure, there was satisfactory color separation. The neutral scale was in var-.- ious densities of blue, that is, cyan plus magenta. The area exposed to red light varied from blue to magenta with increasing red exposure. The area exposed to green light varied from blue to cyan with increasing exposure and the area exposed to blue light varied from blue to white. A spectrogram made from this emulsion showed in the vicinity of 600 millimicrons a small area which was white, indicating that the grains were sensitive both to red and green. Its existence showed that the green-sensitive grains were slightly red-sensitive as the result of diffusion of traces of red-sensitizing dye. This region extends from about 550 to 630 millimicrons. The Wratten No. 70 filter which transmits appreciably only wave lengths'longer than 650 millimicrons was used for the red exposure and accordingly only those grains having the dye inits original form with maximum sensitizing at 670 millimicrons were made developable by the red flash exposure. If the .usual red filter which transmits wave lengths longer than 600 millimicrons had been used for the red re-exposure, there would have been a loss of magenta dye in the regions exposed to red light.

A single layer coating of this type can be converted into a complete three-color coating by overcoating it with an unsensitized emulsion of higher contrast containing a bleachable or otherwise removable yellow dye. This coating is processed in the same manner except that after step 12 there is an exposure to blue light and development in a yellow coupler developer.

It is also possible to have a complete threecolor coating in a single layer without filter dye. Chlorobromide emulsions having sensitivity effectively limited to the violet end of the spectrum are desirable for the red and green sensitive components in this case. The blue .sensitive component may be either a bromoiodide emulsion, or a chlorobromide with optical sensitization. If the blue sensitized chlorobromide emulsion is used, the sensitizer must be nondifiusing only to the extent necessary for color separation in the image exposure of the film. As the flash exposure with blue light is made after the cyan and magenta developments, the red and green sensitive grains all having been developed either in the first or color developers, slight diffusion of the blue sensitizer in development does not introduce the contamination of colors which is caused by diffusion of the red or green sensitizer.

The two layer three-color coatings using a yellow-dyed second coat may be used as camera films or for printing of normal color originals. The single-layer three-color coatings must be exposed through a filter absorbing all radiation to which the un-sensitized chlorobromide emulsion is sensitive. With very low proportions of bromide, this may be a filter such as the Wratten 2A, absorbing radiation of wavelengths less than 420 m but such emulsions are generally quite slow and there is more difliculty with diffusion of sensitizers. It is easier to work with emulsions containing a substantial proportion of bromide, requiring exposure to light only of wavelengths greater than 450 or 460 m Such films may be exposed from separation positives through appropriate filters, or may be used to make prints from colored originals in which the positions of the absorption maxima of the dyes were displaced to longer wavelengths than is normal.

In the accompanying drawing,' Figure 1 is a reproduction of a wedge spectrogram in color of a coating similar to that described in Example 1. As shown in Figure l, the blue spectral region up to about 460 m varies from blue in the regions of low exposure (at the top of the spectrogram) to white in the regions of high exposure. In the region of green exposure from about 460 to 580 m the spectrogram varies from blue in the regions of low exposure to cyan in the region of high exposure. In the region of red exposure from about 580 to 700 m the spectrogram is blue in the regions of low exposure. The red exposure region from about 640 to 700 m is magenta in the high exposure region.

In the regions from about 500 to 540 m and 580 to 640 m there is a white region. These regions indicate that the silver halide grains were exposed by both red and green light. The region at 500 to 540 m indicates wandering of the green sensitizing dye and the region at 580 to 640 m indicates wandering of the red sensitizing dye. These regions show that the red sensitive rains were slightly green sensitive as the result of diffusion of traces of green sensitizing dye and the green sensitive grains were slightly red sensitive as the result of diffusion of traces of red sensitizing dye.

If sensitizing dyes had been used in this coating, which sensitized the grains to which they diiiused in the same region as to that which the originally sensitized grains were sensitive, it is apparent that the regions which are white in the spectogram would have extended throughout the green and red regions respectivelyand would have diluted the colors in these regions.

Since the white portions caused by sensitizer wandering are in a confined region of the spectrum, their effect can be avoided by the use of filters of proper light transmission characteristics. As shown in Figure 2 of the drawing, a filter can be used for the red light re-exposure which iransmits only that light having wavelengths longer than 640 Ill 1.. Below 640 m there is no transmission by this filter and the region of 580 to 640 m to which the diffusing red sensitizing dye sensitizes, is not exposed.

Similarly, in re-exposing the emulsion to green light, a filter can be used, such as that shown in Figure 3 of the drawing which transmits only that light having wavelength longer than about 520 n'l L. Light below this wavelength is not transmitted and for this reason, there is no green light exposure of the region around 520 m, to which the difiusing green sensitizing dye sensitizes.

Example 2 A gelatino-silver bromoiodide emulsion was sensitized as in Example 1 with milligrams per gram mole of silver halide of 3,3-dimethyl-8,l0- toloxythiacarbocyanine bromide as the red sensitizer and 85 milligrams per gram mole of silver halide of 1',3-diethyl-4,5-benz-6-methylthia-2- cyanine bromide as the green sensitizer. It was exposed and processed as in Example 1 except that a Wratten No. 29 filter transmitting wave lengths longer than 610 millimicrons was used instead of Wratten No. '70 filter for the red reexposure.

Example 3 A gelatino-silver chlorobromide emulsion approximately 60% silver chloride and 40% silver bromide containing 0.30 gram moles of silver halide per liter was used. One-half of this emulsion was sensitized with 18 milligrams per liter of emulsion of 3,3-dimethyl-9-phenyl-4,5,4,5- dibenzthiacarbocyanine chloride as the red sensitizer. The other half was sensitized with 25 milligrams per liter of emulsion of 3.3',9-triethyl- 5,5-diphenyloxacarbocyanine bromide (U. S. Patent 2,295,276) as the green sensitizer. The mixedemulsion was exposed and processed as in Example 1 except that the black and white dev op e t (St p 3) was for two minutes instead of 8 minutes. Color separation was satisfactory.

Example 4 An emplsion like that of Example 3 was used. One-third of the emulsion was red sensitized as in Example 3, and one-third green sensitized as in Example 3. The remaining third was sensitized to the blue (maximum at 470 m by adding 25 mg. per liter of 3,3-diethyl-6,I,6,'7'-dibenzthiacyanine chloride (J. Chem. Soc. 1930, page 2508), heating to 50 C. for five minutes, cooling to 35 before mixing with the red and green sensitive portions. It was exposed for formation of the image to red light (Wratten No. 29 filter), green light (Wratten N0. 58 filter) and blue light (Wratten No. 49 and No. 3 filters, the latter to prevent exposure of the red and green sensitive grains). Processing was similar to that of Example 3 except that after step 12 it was exposed uniformly to white light and developed for 10 minutes in a yellow coupler developer of the following composition:

Sodium hexametphosphate grams 0.5 Sodium sulfite do 5.0 2-amino-5-diethylamino toluene hydrochloride do 2.0 Sodium carbonate do 20.0 Potassium bromide do 0.25 w-benzoylacetanilide do 1.0 Sodium hydroxide do 2.0 Water to 1iter 1.0

Following this, it was washed for 10 minutes and processing completed by steps 13-15. Color separation was adequate.

With chlorobromide emulsions, it is important that the emulsion be coated promptly after mixing the two sensitized portions.

Although we have described out invention with particular reference to reversal processes, the principle may also be applied to mixed grain negative-positive processes. Exposures on films designed for such purposes may be made with narrow spectral bands of radiation chosen to correspond with the maxima of the dye in their original condition.

It will be apparent that our invention consists of two principal features (1) the use in mixed grain emulsions for color processes of a sensitizing dye which at low concentrations has a maximum of sensitization appreciably shorter than that which is characteristic of the normal concentration used in sensitizing the grains and (2) exposure of the emulsion at appropriate steps in the process by radiation corresponding to the normal maximum of sensitization and excluding or minimizing that corresponding to sensitization by low concentrations of dye which has diffused to other grains.

What we claim is:

1. The method of selectively exposing the difierently-sensitized silver halide grains of a mixed grain photographic layer having silver halide grains sensitized in one spectral region with a sensitizing dye that diffuses slightly between silver halide grains and produces a different sensitizing maximum on the grains to which it diffuses than on the grains originally sensitized, and having at least one other type of silver halide grains sensitized in a spectral region difierent from said first-mentioned spectral region and also sensitized to some extent by said dye which diffuses slightly, which comprises exposing said layer with light of the spectral region to which said first-mentioned grains were originally sensitized while excluding light of the spectral region to which said other type of grains are sensitized by diffusion of said dye, and also exposing said layer to light to which said other type of grains are sensitive, and developing differently colored images with said two types of grains.

2. The method of selectively exposing the dif ferently-sensitized silver halide grains of a mixed grain photographic layer having silver halide grains sensitized in one spectral region with a sensitizing dye that diffuses slightly between silver halide grains and produces a sensitizing maximum on the grains to which it diffuses at shorter wave length than on the grains originally sensitized and having at least one. other type of silver halide grains sensitized in a spectral region different from said first-mentioned spectral region and also sensitized to some extent by said dye which diffuses slightly, which comprises exposing said layer to a subject, developing a negative silver image in said layer, uniformly exposing said layer with light of the spectral region to which first-mentioned grains were originally sensitized while excluding light of the spectral region to which said other type of grains are sensitized by diffusion of said dye and developing a colored image in the region of said grains so exposed, and then uniformly exposing said other grains to light to which they are sensitive and developing an image of a different color in the region of said other grains.

3. The method of selectively exposing the differently sensitized silver halide grains of a mixed grain photographic layer having silver halide grains sensitized to red light with a sensitizing dye that diffuses slightly between silver halide grains and produces a sensitizing maximum on the grains to which it diffuses at shorter wave length than on the grains originally sensitized, said silver halide grains being also sensitized to some extent with said after-mentioned greensensitizing dye, and having other silver halide grains sensitized to green light with a sensitizing dye that diffuses slightly between silver halide grains and produces a sensitizing maximum on the grains to which it diffuses at shorter wave length than on the grains originally sensitized, said other silver halide grains being also sensitized to some extent with said red-sensitizing dye, which comprises exposing said layer to a subject, developing negative silver images in said layer, uniformly exposing said layer with red light of the region to which said red-sensitive grains were originally sensitized while excluding light of the spectral region to which said greensensitizing grains were sensitized by diffusion of said red sensitizing dye, developing a cyan image in the region of the grains so exposed, uniformly exposing said layer with green light of the spectral region to which the silver halide grains were originally sensitized by said green-sensitizing dye while excluding light of the spectral region to which said grains were sensitized by diffusion of said red-sensitizing dye and developing a magenta image in the region of said green-sensitized grains.

BURT H. CARROLL. WESLEY T. HANSON, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,980,941 Mannes et al Nov. 13, 1934 2,252,718 Mannes et al Aug. 19, 1941 2,318,597 Davey et al. May 11, 1943 2,320,418 Eggert et a1 June 1, 1943 2,388,859 Mannes et al Nov. 13, 1945 

1. THE METHOD OF SELECTIVELY EXPOSING THE DIFFERENTLY-SENSITIZED SILVER HALIDE GRAINS OF A MIXED GRAIN PHOTOGRAPHIC LAYER HAVING SILVER HALIDE GRAINS SENSITIZED IN ONE SPECTRAL REGION WITH A SENSITIZING DYE THAT DIFFUSES SLIGHTLY BETWEEN SILVER HALIDE GRAINS AND PRODUCES A DIFFERENT SENSITIZING MAXIMUM ON THE GRAINS ORIGINALLY SENSITIZED, FUSES THAN ON THE GRAINS ORIGINALLY SENSITIZED, AND HAVING AT LEAST ONE OTHER TYPE OF SILVER HALIDE GRAINS SENSITIZED IN A SPECTRAL REGION DIFFERENT FROM SAID FIRST-MENTIONED SPECTRAL REGION AND ALSO SENSITIZED TO SOME EXTENT BY SAID DYE WHICH DIFFUSES SLIGHTLY, WHICH COMPRISES EXPOSING SAID LAYER WITH LIGHT OF THE SPECTRAL REGION TO WHICH SAID FIRST-MENTIONED GRAINS WERE ORIGINALALLY SENSITIZED WHILE EXCLUDING LIGHT OF THE SPECTRAL REGION TO WHICH SAID OTHER TYPE OF GRAINS ARE SENSITIZED BY DIFFUSION OF SAID DYE, AND ALSO EXPOSING SAID LAYER TO LIGHT TO WHICH SAID OTHER TYPE OF GRAINS ARE SENSITIVE, AND DEVELOPING DIFFERENTLY COLORED IMAGES WITH SAID TWO TYPES OF GRAINS. 